Method for Starting a Grinding Tube

ABSTRACT

A method for starting a grinding tube with an assigned drive device, wherein during the operation of the grinding tube a grinding mode and a charge release mode can be set such that a particularly reliable monitoring of the state of charge located in the grinding tube is ensured, where the grinding tube is rotated and, at a first rotational angle, a first actual torque is detected, a setpoint torque is calculated for a second, relatively large rotational angle based on the first actual torque, an actually occurring, second actual torque is detected when the second rotational angle is reached, an investigation is performed to determine the difference of the second actual torque from the setpoint torque, and the charge release mode of the grinding tube is set when the second actual torque is within the threshold range, otherwise the grinding tube is operated in the grinding mode.

The invention relates to a method for starting a grinding tube with an allocated drive apparatus, wherein during operation of the grinding tube a grinding mode and a charge release mode can be set. The invention further relates to a control apparatus, a drive apparatus and also a tube mill.

Tube mills are preferably used for grinding materials such as ore. It is not unusual for the operation of a tube mill to be interrupted for a longer period of time and for the tube mill to be at a standstill. This happens due to maintenance reasons, for example. During the standstill of the tube mill, material located in the grinding tube of the tube mill can solidify and stick to the interior wall of the grinding tube. Material which has stuck, solidified and is adhering to the interior wall of the grinding tube in this manner is referred to as a “frozen charge”. If the tube mill is brought back into operation after a longer standstill, there is the danger that the frozen charge detaches from the interior wall at a great height, falls and causes considerable damage to the tube mill when it subsequently strikes against the grinding tube.

A method for removing a frozen charge from the interior wall of a grinding tube emerges from EP 1 735 099 B1, in which the angle of rotation is set by a drive apparatus to oscillate by at least a predefined angle of rotation.

Monitoring facilities or monitoring functions in the controller of the tube mill already exist, which detect the presence of frozen charges and which shut down the tube mill when the presence of a frozen charge is detected. Such a monitoring of the load state of a tube mill is described for example in the unexamined German patent application DE 35 28 409 A1.

In the case of such a monitoring function, the torque required when starting up the tube mill can be continuously observed and the maximum value of the course is stored. When reaching the set angle of rotation of the monitoring (e.g. 70°), the current torque is compared with the stored peak value for a following time window. If the torque in the monitoring window is greater than 95% of the stored maximum value, this implies a caked-on charge (in which the torque is continuously increased up to an angle of rotation of 90°) and the tube mill is shut down.

Particularly in the case of mills which are operated without using steel balls to aid grinding (AG mills, autogenous mills), the charge is relatively fluid, meaning that no pronounced torque peaks emerge on startup. Thus, the condition for shutdown is fulfilled by the monitoring, however, although there is no caked-on charge present. The mill is shut down nonetheless.

An automatic deactivation of the monitoring function described above does not make sense, however, as even in these mills there is the danger that the water located in the grinding tube drains off during standstill and the material subsequently cakes on. It is therefore not possible to dispense with the monitoring.

The invention is based on the object of ensuring a particularly reliable monitoring of the status of the material located in a grinding tube of a tube mill.

The object is achieved according to the invention by a method for starting a grinding tube with an allocated drive apparatus, wherein during operation of the grinding tube a grinding mode and a charge release mode can be set, wherein, starting from a standstill state of the grinding tube:

-   -   the grinding tube is rotated and, at a first angle of rotation,         a first actual torque is acquired,     -   on the basis of the first actual torque a target torque is         calculated for a second, greater angle of rotation,     -   when the second angle of rotation is reached, an effective         second actual torque is acquired,     -   with the aid of a predefined threshold range, the extent by         which the second actual torque deviates from the target torque         is examined,     -   if the second actual torque lies within the threshold range, the         charge release mode of the grinding tube is set,     -   otherwise, the grinding tube is operated in grinding mode.

The object is further achieved according to the invention by a control apparatus for the drive apparatus of a grinding tube for carrying out the method.

Moreover, the object is achieved according to the invention by a drive apparatus for a grinding tube, comprising a control apparatus of this kind.

Finally, the object is achieved according to the invention by a tube mill comprising a grinding tube and a drive apparatus of this kind.

The advantages and preferred embodiments disclosed below in relation to the method can be applied accordingly to the control apparatus, the drive apparatus and the tube mill.

The grinding mode is understood in this context as the normal operation of a tube mill, in which the grinding tube is rotated in view of crushing or pulverizing the charge.

The charge release mode is understood as an operating state of the tube mill in which, if a frozen charge is detected, measures for removing the frozen charge from the interior wall of the grinding tube are initiated. Such measures can follow the automated modes of operation described in EP 1 735 099 B1 and EP 2 525 914 B1. Alternatively, for example, there may only be provision for a shutdown of the rotation of the grinding tube in view of manually removing the frozen charge from the interior wall.

The invention is based on the consideration of checking, by acquiring the torque of the drive apparatus at two different angles of rotation, whether the charge of the grinding tube is a “sliding” or frozen material. In this context, the acquired torque is the drive torque (or alternatively the load moment) of the grinding tube. In principle, it holds that in the event of the entire charge forming a “frozen charge”, at an angle of rotation of less than 90° the torque of the drive apparatus climbs constantly, as long as the charge adheres to the interior wall of the grinding tube. If, however, parts of the charge come loose on startup of the grinding tube and others still continue to adhere to the interior wall, the torque increase over time or as the angle of rotation increases is less than in the case of the total frozen charge.

Based on this knowledge, according to the invention the torque is acquired at two angles of rotation when starting up the grinding tube. In the case of the first, smaller angle of rotation, comparatively less of the charge has slid down on startup of the grinding tube. On the basis of said first actual torque, it is calculated which torque is to be expected at the second, higher angle of rotation if the conditions in the grinding tube no longer change. The effective actual torque for the second angle of rotation is subsequently likewise acquired; as a rule, it lies below the extrapolated target value, since more material usually comes loose from the interior wall with increasing height of the charge during the further rotation of the grinding tube.

Finally, use is made of the threshold range, which makes a statement as to the extent by which the actual torque deviates from the target torque. If the second actual torque lies significantly below the extrapolated target torque, outside of the threshold range, it is assumed that the charge has for the most part or even completely come loose from the interior wall of the grinding tube and the normal grinding operation of the tube mill can be continued. If, however, the second actual torque lies within the threshold range, this means that the material is still adhering to the interior wall of the grinding tube for the most part. The charge release mode is therefore set, meaning that the charge is released from the wall before the grinding tube is rotated further.

By suitable selection of the threshold range, a very high reliability of the “frozen charge” detection is ensured, whereby the availability of the tube mill is likewise increased, since unnecessary shutdowns and thus production outages are avoided. A significant advantage of the proposed method is that the method is independent of a maximum value of the course of the torque. In addition, the expense for the realization of the method is minimal, since as a rule all necessary measured variables are available in any case; they are merely implemented in a further software function in the controller of the tube mill. Moreover, the method is independent of the direction of rotation.

According to a preferred development, to calculate the target torque, use is made of the sine of the first angle of rotation and the sine of the second angle of rotation, in particular the ratio of the sine of the first angle of rotation to the sine of the second angle of rotation. This is particularly advantageous since in the case of caked-on grinding product the torque essentially rises according to a sine of the angle of rotation. A calculation of the target torque in the second angle of rotation, starting from the first angle of rotation, on the basis of a sine-based extrapolation thus supplies the most accurate result for the target torque.

In accordance with a preferred embodiment, a quotient is formed from the second actual torque and the target torque. Such a quotient offers a particularly simple option for establishing a relationship between the two values, in order to examine how these interrelate or how great the difference is between the two. Alternatively to the quotient, for example, the difference is formed from the second actual torque and the target torque and this can likewise be compared with a predefined threshold range or threshold value.

In accordance with a further preferred embodiment, the threshold range is defined by a value which in particular is specified as a percentage or as a rational number. In this context, the value forms the number boundary for the ratio of the target torque and the second actual torque to one another. In the case where a quotient is worked with when evaluating the actual torque against the target torque, said quotient is always less than 1 if the actual torque is in the numerator and the target torque is in the denominator. In the opposite scenario, if in the case of the quotient the target torque is in the numerator and the actual torque is in the denominator, the quotient is always greater than 1. The threshold range is also selected accordingly.

Advantageously, the threshold range is defined as a deviation of the actual torque from the target torque by 15%, in particular by 10%, in particular by 5%. In order to avoid unnecessary faults in the operation of the grinding tube, in this context the threshold range is selected such that the charge release mode is only initiated if the deviation of the second actual torque from the target torque is minimal, which is a sign that on rotation of the grinding tube between the first angle of rotation and the second angle of rotation barely any material has comes loose from the interior wall of the grinding tube.

Preferably, the first and the second angle of rotation lie below 90°, in particular below 70°. According to the invention, with an angle of rotation of approx. 70°, the charge release mode is initiated in order to avoid the grinding product falling onto the bottom of the grinding tube; the aforementioned checking is therefore carried out in an angular range of below 70°.

An exemplary embodiment of the invention will be described in greater detail with reference to a drawing. In the figures:

FIG. 1 shows a schematic and greatly simplified diagram of a grinding tube in four different angles of rotation,

FIG. 2 shows an evaluation of the torque of the grinding tube in accordance with FIG. 1 as a function of an angle of rotation.

The same reference characters have the same meaning in the figures.

In FIG. 1 , a grinding tube 2 of a tube mill not shown in more detail here is represented symbolically. Allocated to the grinding tube 2 is a drive apparatus 3 with a control apparatus 4, which actuates inter alia the starting of the grinding tube 2. The grinding tube 2 is charged with a grinding product 6, in particular ore, which is furthermore referred to as charge.

The grinding tube 2 can be operated both in a grinding mode and also in a charge release mode. The grinding mode represents the normal operation of a tube mill, in which the grinding tube is rotated in view of crushing or pulverizing the charge. The charge release mode is the operating state of the tube mill in which, if a frozen charge is detected, measures for removing the frozen charge from the interior wall of the grinding tube are initiated.

In FIG. 1 , a total of four states of the charge 6 of the grinding tube 2 are shown according to angle of rotation. Z₁ represents a standstill state at 0°, in which the charge 6 is evenly distributed on the bottom on the grinding tube 2. Z₄ represents the position of the caked-on charge at an angle of rotation of approx. 70°. Z₂ and Z₃ represent the caked-on charge 6 at two further angles of rotation α₁, α₂ between 0° and 70°.

Starting from the standstill state Z₁, the operation of the tube mill is commenced and the grinding tube 2 is driven in the direction of rotation 10, in that it is rotated about a central axis A. At a first angle of rotation α₁, which is smaller than 90°, for example at 45°, a torque T of the drive apparatus 3 is measured. This point is represented by M1 in FIG. 1 . At a second angle of rotation α₂, e.g. 60°, the torque T is measured again at point M2. The measurements can also take place at other angles of rotation D between 0° and 90°; only at least two measured values at two different angles of rotation are required.

The evaluation of the measurements at the measurement points M1 and M2 is shown graphically in FIG. 2 . In this context, the torque T of the drive apparatus 3 is plotted as a function of the angle of rotation D. V₁ refers to the increase in the torque T with a frozen charge. V₂ is the effective course of the torque T when starting the tube mill. M refers to the maximum torque occurring.

In the region B, which is used in the prior art for monitoring the “frozen charge”, lies in the exemplary embodiment shown a minimum deviation of the effective course V₂ of the torque T from the maximum torque M. In this context, the course V₂ has no pronounced torque peaks. At this point, conventional monitoring systems would thus regularly initiate the charge release mode.

In order to avoid this, the torque T at the measurement points M1 and M2 is determined at α₁ and α₂, respectively. With caked-on grinding product 6, the torque T largely increases according to a sine of the angle of rotation D, as can be seen from the course of V₁. It is therefore possible to determine from the torque T1 at the first angle of rotation (measurement M1), using the relationship

sin (α₁)/sin (α₂),

the theoretical target torque T_(2TARGET) at the point in time M2 at angle of rotation α₂.

The effective torque T_(2ACTUAL) at the measurement point M2 at α₂ is measured in addition and compared with T_(2TARGET), by making use of a threshold range 8. In the exemplary embodiment shown, the threshold range is defined as 10%, i.e. it is examined whether T_(2ACTUAL) deviates more than 10% from T_(2TARGET). If T_(2ACTUAL) is more than 10% below T_(2TARGET) or is equal to T_(2TARGET), it should be assumed that the material has come loose from the interior wall of the grinding tube 2 and the tube mill is continued to be operated without faults. Otherwise, for example, the charge release mode is set, in particular the tube mill is shut down or use is made of a controlled rattling or shaking of the grinding tube 2 by way of adapting the drive torque.

To compare T_(2TARGET) with T_(2ACTUAL), the threshold range 8 is stored in the controller 4 or use is made thereof on demand in a case-related manner. For the evaluation, in particular the quotient of T_(2ACTUAL) and T_(2TARGET) is formed and this is compared with the threshold range 8. In the above exemplary embodiment, in which the deviation boundary is defined at 10%, the condition for initiating the charge release mode is fulfilled when

T _(2ACTUAL) <T _(2TARGET)×0.9.

As

T _(2TARGET) =T ₁(sin (α(₁)/sin (α₂)),

the following applies:

T _(2ACTUAL) <T ₁(sin (α₁)/sin (α₂))×0.9.

With the angles of rotation α₁=45° and α₂=60° used in accordance with FIG. 1 , the condition for the charge release mode can thus be expressed mathematically by:

T _(2ACTUAL) <T ₁×1.1.

If T_(2ACTUAL) is equal to or greater than T_(2TARGET) by 10%, however, then the normal grinding mode is continued. 

1. A method for starting a grinding tube (2) with an allocated drive apparatus (3), wherein during operation of the grinding tube (2) a grinding mode and a charge release mode can be set, wherein, starting from a standstill state of the grinding tube (2): the grinding tube (2) is rotated and, at a first angle of rotation (α₁), a first actual torque (T₁) is acquired, on the basis of the first actual torque (T₁) a target torque (T_(2TARGET)) is calculated for a second, greater angle of rotation (α₂), when the second angle of rotation (α₂) is reached, an effective second actual torque (T_(2ACTUAL)) is acquired, with the aid of a predefined threshold range (8), the extent by which the second actual torque (T_(2ACTUAL)) deviates from the target torque (T_(2TARGET)) is examined, if the second actual torque (T_(2ACTUAL)) lies within the threshold range (8), the charge release mode of the grinding tube (2) is set, otherwise, the grinding tube (2) is operated in grinding mode.
 2. The method as claimed in claim 1, wherein, to calculate the target torque (T_(2TARGET)), use is made of the sine of the first angle of rotation (α₁) and the sine of the second angle of rotation (α₂), in particular the ratio of the sine of the first angle of rotation (α₁) to the sine of the second angle of rotation (α₂) is formed.
 3. The method as claimed in one of the preceding claims, wherein a quotient is formed from the second actual torque (T_(2ACTUAL)) and the target torque (T_(2TARGET)).
 4. The method as claimed in one of the preceding claims, wherein the threshold range (8) is defined by a value which in particular is specified as a percentage or as a rational number.
 5. The method as claimed in claim 4, wherein the threshold range (8) is defined as a deviation of the second actual torque (T_(2ACTUAL)) from the target torque (T_(2TARGET)) by 15%, in particular by 10%, in particular by 5%.
 6. The method as claimed in one of the preceding claims, wherein the first and the second angle of rotation (α₁, α₂) lie below 90°, in particular below 70°.
 7. A control apparatus (4) for the drive apparatus (3) of a grinding tube (2) for carrying out the method as claimed in one of the preceding claims.
 8. A drive apparatus (3) for a grinding tube (2), comprising a control apparatus (4) as claimed in claim
 7. 9. A tube mill comprising a grinding tube (2) and a drive apparatus (3) as claimed in claim
 8. 